Min Hwan Jeon

Controllable Nondegenerate p-Type Doping of Tungsten Diselenide by Octadecyltrichlorosilane

Despite heightened interest in 2D transition-metal dichalcogenide (TMD) doping methods for future layered semiconductor devices, most doping research is currently limited to molybdenum disulfide (MoS2), which is generally used for n-channel 2D transistors. In addition, previously reported TMD doping techniques result in only high-level doping concentrations (degenerate) in which TMD materials behave as near-metallic layers. Here, we demonstrate a controllable nondegenerate p-type doping (p-doping) technique on tungsten diselenide (WSe2) for p-channel 2D transistors by adjusting the concentration of octadecyltrichlorosilane (OTS). This p-doping phenomenon originates from the methyl (-CH3) functional groups in OTS, which exhibit a positive pole and consequently reduce the electron carrier density in WSe2. The controlled p-doping levels are between 2.1 × 10(11) and 5.2 × 10(11) cm(-2) in the nondegenerate regime, where the performance parameters of WSe2-based electronic and optoelectronic devices can be properly designed or optimized (threshold voltage↑, on-/off-currents↑, field-effect mobility↑, photoresponsivity↓, and detectivity↓ as the doping level increases). The p-doping effect provided by OTS is sustained in ambient air for a long time showing small changes in the device performance (18-34% loss of ΔVTH initially achieved by OTS doping for 60 h). Furthermore, performance degradation is almost completely recovered by additional thermal annealing at 120 °C. Through Raman spectroscopy and electrical/optical measurements, we have also confirmed that the OTS doping phenomenon is independent of the thickness of the WSe2 films. We expect that our controllable p-doping method will make it possible to successfully integrate future layered semiconductor devices.

High-Performance 2D Rhenium Disulfide (ReS2) Transistors and Photodetectors by Oxygen Plasma Treatment

A high-performance ReS2 -based thin-film transistor and photodetector with high on/off-current ratio (10(4) ), high mobility (7.6 cm(2) V(-1) s(-1) ), high photoresponsivity (2.5 × 10(7) A W(-1) ), and fast temporal response (rising and decaying time of 670 ms and 5.6 s, respectively) through O2 plasma treatment is reported.

Wide-range controllable n-doping of molybdenum disulfide (MoS2) through thermal and optical activation.

Despite growing interest in doping two-dimensional (2D) transition metal dichalcogenides (TMDs) for future layered semiconductor devices, controllability is currently limited to only heavy doping (degenerate regime). This causes 2D materials to act as metallic layers, and an ion implantation technique with precise doping controllability is not available for these materials (e.g., MoS2, MoSe2, WS2, WSe2, graphene). Since adjustment of the electrical and optical properties of 2D materials is possible within a light (nondegenerate) doping regime, a wide-range doping capability including nondegenerate and degenerate regimes is a critical aspect of the design and fabrication of 2D TMD-based electronic and optoelectronic devices. Here, we demonstrate a wide-range controllable n-doping method on a 2D TMD material (exfoliated trilayer and bulk MoS2) with the assistance of a phosphorus silicate glass (PSG) insulating layer, which has the broadest doping range among the results reported to date (between 3.6 × 10(10) and 8.3 × 10(12) cm(-2)) and is also applicable to other 2D semiconductors. This is achieved through (1) a three-step process consisting of, first, dopant out-diffusion between 700 and 900 °C, second, thermal activation at 500 °C, and, third, optical activation above 5 μW steps and (2) weight percentage adjustment of P atoms in PSG (2 and 5 wt %). We anticipate our widely controllable n-doping method to be a starting point for the successful integration of future layered semiconductor devices.

A High-Performance WSe2 /h-BN Photodetector using a Triphenylphosphine (PPh3 )-Based n-Doping Technique.

The effects of triphenylphosphine (PPh3 )-based n-doping and hexagonal boron nitride (h-BN) insertion on a tungsten diselenide (WSe2 ) photodetector are systematically studied, and a very high performance WSe2 /h-BN heterostucture-based photodetector is demonstrated with a record photoresponsivity (1.27 × 10(6) A W(-1) ) and temporal photoresponse (rise time: 2.8 ms, decay time: 20.8 ms) under 520 nm wavelength and 5 pW power laser illumination.

A study on the etching characteristics of magnetic tunneling junction materials using DC pulse-biased inductively coupled plasmas

The etching properties of magnetic materials composing the magnetic tunnel junction (MTJ) such as CoPt, MgO, CoFeB, and CoPt/MgO/CoFeB were investigated in DC pulse biased CO/NH3 inductively coupled plasmas (ICPs) and their etch characteristics were compared with those etched by RF CW biased ICPs. The use of DC pulse biased ICPs instead of RF CW biased ICPs improved the etch selectivity of the MTJ materials over W and also decreased the residue on the surface of the etched materials possibly due to the more stable and volatile etch product formation during the DC pulse off time and the enhanced removal of the etch products by mono-energetic ions during the DC pulse on time. When MTJ materials masked with W were etched, more anisotropic etch profile could be also observed when the MTJ materials were etched with the DC pulse biasing of 60% duty percentage compared with those etched with RF CW biasing due to the decreased redeposition of etch products on the sidewall of the etched feature in addition to the enhanced etch selectivity over W.

Selective Etching of Magnetic Tunnel Junction Materials Using CO/NH3 Gas Mixture in Radio Frequency Pulse-Biased Inductively Coupled Plasmas

To enhance the volatility of etch products and to increase the etch rates of MTJ materials, other techniques such as substrate heating and source power RF pulsing have been also investigated. 26–28) The study on the effect of substrate temperature during the etching of the MTJ materials using CH3OH in an ICP showed that, with the increase of substrate temperature from 20 to 120 � C, the deposition of sidewall residue was decreased while the etch rates of MTJ materials were increased. In the case of etching the MTJ materials using the pulsing of microwave plasma in Cl2-based gases (source power pulsing not bias power pulsing), high etch rates of MTJ materials without corrosion or delamination were observed, while corrosion and delamination of MTJ materials were observed in the etching using continuous wave (CW) microwave plasmas. The researchers believe that the negative ions formed during the power-off period enhance the chemical reactions on the surface of magnetic films. They reported that the magnetic characteristics were also significantly improved by using the source powerpulsed plasma because of reduced residues in addition to the improvement of the etch profile. 29)

Etch characteristics of magnetic tunnel junction materials using substrate heating in the pulse-biased inductively coupled plasma

Magnetic tunnel junction (MTJ)-related materials such as CoFeB, MgO, and W were etched in a pulse-biased inductively coupled plasma etch system using a CO/NH3 gas combination, and the effects of substrate temperature (room temperature ∼200 °C) in the pulse-biased condition on the etch characteristics of the MTJ-related material were investigated. The etch selectivity of MTJ materials over W was improved by substrate heating possibly due to the easy removal of the compounds from the etched CoFeB surface during the pulse-on time at the elevated substrate temperature. At high substrate temperature, decreased thickness of etch residue was observed not only on the bottom surface but also on the sidewall surface during the etching, which indirectly indicated the increased volatility of the etch compounds at higher substrate temperature. The etching of CoFeB features masked with W also showed a more anisotropic etch profile by heating the substrate up to 200 °C possibly due to the increased the etch selectivity ...

Etch residue removal of CoFeB using CO/NH3 reactive ion beam for spin transfer torque-magnetic random access memory device

Using a reactive ion beam etching (RIBE) system, the possibility of removing the sidewall residues remaining on etched nanoscale CoFeB features and the W hard mask after using a conventional inductively coupled plasma etching system was investigated. Upon increasing the ion energy of the Ar beam, a similar sputter yield increase was found for both CoFeB side wall residues and the W hard mask. Hence, increasing the ion beam energy to improve etch residue removal efficiency at the same time induces a degradation of the CoFeB profile because of the W hard mask erosion. However, when CO/NH3 was used as the RIBE gas mixture, at ion energy in the range of 90–110 eV, the effective residue removal from CoFeB etched features without etching the W hard mask. When the ion energy of the CO/NH3 RIBE exceeds 140 eV, again similar sputter yields are found for both CoFeB side wall residues and the W hard mask.

Layer-controlled thinning of black phosphorus by an Ar ion beam

Black phosphorus (BP) is one of the most interesting two-dimensional (2D) layered materials due to its unique properties, including a band gap energy change from 0.3 eV (bulk) to 2.0 eV (monolayer) depending on the number of BP layers, for application in nanoelectronic devices. In general, 2D layered materials including BP have limitations in terms of synthesis due to the process factors such as time, temperature, etc., and thus, a thinning technique from the bulk material to a 2D material needs to be used while controlling the removed layer thickness. In this study, layer-controlled thinning of BP was performed by using a controlled Ar+ ion beam method and the BP thinning characteristics were investigated. By using the near monoenergetic ion energy in the range of 45–48 eV, BP could be thinned with the thinning rate of ∼0.55 nm min−1 down to bilayer BP without increasing the surface roughness and without changing the chemical binding states. The BP oxide on the pristine BP could also be successfully removed using the same Ar+ ion beam. 2D BP field-effect transistors (FETs) fabricated with the thinned bilayer–10-layer BPs exhibited electrical characteristics similar to those of pristine BP FETs suggesting no electrical damage on the BP layers thinned by the controlled monoenergetic Ar+ ion beam.

Effect of source frequency and pulsing on the SiO2 etching characteristics of dual-frequency capacitive coupled plasma

A SiO2 layer masked with an amorphous carbon layer (ACL) has been etched in an Ar/C4F8 gas mixture with dual frequency capacitively coupled plasmas under variable frequency (13.56–60 MHz)/pulsed rf source power and 2 MHz continuous wave (CW) rf bias power, the effects of the frequency and pulsing of the source rf power on the SiO2 etch characteristics were investigated. By pulsing the rf power, an increased SiO2 etch selectivity was observed with decreasing SiO2 etch rate. However, when the rf power frequency was increased, not only a higher SiO2 etch rate but also higher SiO2 etch selectivity was observed for both CW and pulse modes. A higher CF2/F ratio and lower electron temperature were observed for both a higher source frequency mode and a pulsed plasma mode. Therefore, when the C 1s binding states of the etched SiO2 surfaces were investigated using X-ray photoelectron spectroscopy (XPS), the increase of C–Fx bonding on the SiO2 surface was observed for a higher source frequency operation similar to a pulsed plasma condition indicating the increase of SiO2 etch selectivity over the ACL. The increase of the SiO2 etch rate with increasing etch selectivity for the higher source frequency operation appears to be related to the increase of the total plasma density with increasing CF2/F ratio in the plasma. The SiO2 etch profile was also improved not only by using the pulsed plasma but also by increasing the source frequency.